Some LED technologies are very susceptible to electrostatic discharge or ESD events. To improve their reliability in manufacturing and in the field, some LEDs will come with a Zener diode in parallel with the LED die. This photo illustrates three LED dice and their bond wires. The lower left hand die is a small Zener diode that provides ESD protection to the three LED dice. These devices still should have a PLED device for protection. This tiny Zener die may provide effective ESD protection, but it cannot handle the power required to bypass the current around an open LED. With only the Zener diode for protection, a failed wire bond to the LED die will force too much power into the Zener and it will quickly be destroyed and likely cause its wire bond to fail as well. External Zener or TVS diodes are never a good choice to compete with a PLED device because they are clamping devices which means their on state voltage is very high. When forced into operation by an open LED, they will generate considerable heat, raising the temperature of the total fixture. In contrast, the PLED is a crowbar device and has a very low on state voltage. It will generate lower heat than a healthy LED, so the temperature of the total fixture will actually decrease when the PLED is activated.